1. Field of the Invention
The present invention relates to a combustion apparatus for a vehicle and, more particularly, to a combustion apparatus for removing fine particles existing in exhaust gas collected by a filter, for example, provided in an exhaust gas passage of a diesel engine.
2. Description of the Prior Art
The present invention contemplates to efficiently regenerate a filter so as to remove fine particles existing in the exhaust gas of a diesel engine as described above, since there is no apparatus of this type at present. It is desirable to develop such an apparatus for the following reasons.
The fine particles contained in the exhaust gas of diesel engine are harmful to human's health, and, accordingly, the quantity of exhausted fine particles from the diesel engine may be suppressed by law.
Two means are considered as a method for reducing the quantity of exhausted fine particles from a diesel engine. One is to reduce the quantity of the exhausted fine particles by improving the diesel engine itself. This is ideal, but this method can be expected to reduce only a small amount of the fine particles exhausted from the diesel engine at present, and cannot be relied on to sufficiently reduce the fine particles if the law for suppressing the exhausted fine particles will be severely stipulated. The other method is to filter the fine particles by providing a filter, for example, of ceramic honeycomb in the exhaust gas system of the diesel engine. This method has a drawback that, to prevent the filter from clogging, the filter has to be regenerated. The regeneration of this filter is based on the following principle.
The main ingredient of fine particles which adheres to the filter is carbon. The ignition temperature of carbon is approx. 550.degree. C. Therefore, when the exhaust gas of the engine is heated to temperatures higher than 550.degree. C., the fine particles become carbon dioxide so that the filter can be regenerated. However, the temperature of the exhaust gas of a diesel engine used in ordinary automobiles when operated at normal speeds is approx. 400.degree. C. at the maximum value, and accordingly, it is impossible to regenerate the filter. Thus, it is required to add a certain engine exhaust gas heater, which is used to regenerate the filter. It has been considered as a regenerator for the filter to raise the temperature of the exhaust gas by throttling the air supply to the engine or to provide a regenerative burner, i.e., a combustion apparatus. The regenerative burner is possible at present, but the burner has such drawbacks or disadvantages that the temperature of a burner nozzle, that is, a fuel injection nozzle, is not only raised by means of thermal radiation from the burning flame and thermal conduction from a combustion chamber at the regenerative time, but is also continuously raised by thermal conduction from the high temperature of the air enclosed in the combustion chamber as well as the high temperature of the combustion chamber itself when a shut-off valve for preventing the reverse flow of the exhaust gas is closed during the regeneration or immediately after the termination of the regeneration process whereby, the temperature of the air in the vicinity of the spraying port of the nozzle becomes 100.degree. C. or higher, with the result that unburned fuel adheres to the end face of the nozzle, the adhered unburned fuel being readily converted into tar and hence causing the nozzle to be clogged during or after the termination of the regeneration process.